Transducers are devices which function generally to convert an input of one form into an output of another form or magnitude. Many types of transducers are available for converting light to electrical signals, mechanical energy to electrical signals, temperature to pressure, pressure to electrical signals, etc., and vice versa. In other instances, transducers can simply modify an input stimulus, such as change an input pressure to another pressure, an input amplitude to a different output amplitude, etc. Equipment or apparatus which operates between different types of energy generally requires one type of transducer or another. Based upon the application, transducers can range from inexpensive to very expensive, depending on the precision, accuracy, reliability, etc. required
In the present market, the largest number of pressure to voltage conversion devices (pressure transducers) are piezoresistive. These devices are strain sensitive rather than displacement sensitive. However, for pressure ranges less than a psi FS (Full Span), capacitive displacement transducers are predominantly employed. The capacitance is sensed by electrical circuits to provide an output voltage corresponding to the change in pressure. Highly developed silicon semiconductor processing techniques have given rise to the use of such material as flexible clamped transducer diaphragms which move in response to pressure and provide an output change in electrical resistance or capacitance. Such transducers are disclosed in U.S. Pat. Nos. 4,495,820; 4,424,713; 4,390,925 and 4,542,435. The use of silicon semiconductor material as a pressure transducer diaphragm is a relatively new innovation, and thus many fabrication and operational uncertainties still exist For example, the characterization and mathematical modeling of silicon, when employed as a flexible membrane, is not well known. Therefore, it has been difficult to predict silicon diaphragm transducer performance which exhibits predefined characteristics over a wide range of applications. In addition, the fabrication of such type of transducer has been accompanied by many problems, one of which is that such a conductive diaphragm of the clamped type must be fixed or anchored at its peripheral edge, and electrically insulated from spaced capacitor plates Such an interface requires different materials with the attendant problems of attachment, relative temperature expansion, uniform spacing, stability, reliability and a host of other concerns
The demand for pressure transducers with increased sensitivity has always been high because the market for flow transducers using differential pressure techniques has always been larger than the entire pressure transducer market. This market has been largely unserved for lack of low cost, high sensitivity differential pressure transducers. Recently, this already high demand has been growing because energy conservation and control needs in the heating ventilation and air conditioning area (HVAC) have required more sensitive, precise and accurate flow transduction at a lower cost. In capacitive displacement type transducers, such increased sensitivity is generally achieved by increasing the diameter of the flexible diaphragm. As a result, large diaphragm diameters have been utilized to provide the desired sensitivity. It can be appreciated that with such a construction, such transducers are costly, not easily mass produced and require a lot of space for implementation. In addition, with large diameter diaphragms, the use of silicon makes the transducer economically impractical. Among other disadvantages of many large, metal diaphragm transducers is that an overpressure, even momentarily, often stretches or completely damages the diaphragm and renders the transducer unusable The typical overpressure (proof pressure) for psi ranged transducers is from 1 to 10 times FS. The required overpressure for high sensitivity differential pressure transducers in flow applications is often 10 to 100 times FS and occasionally 1000 times FS. These needs cannot be met by large diameter metal diaphragm devices with proportionally larger gaps.
It can be seen that a need exists for a miniature pressure transducer of the capacitive displacement type which is cost effective and mass producible employing new silicon fabrication and micromachining processes at a wafer level. A need also exists for a miniature transducer, and method of fabrication thereof, in which the sensitivity is not compromised based on size, and in which a large number of transducers can be fabricated in a batch process utilizing semiconductor masking, patterning and deposition technology. There also exists a need for a highly sensitive, small diameter diaphragm which can be overpressured without damage to the unit.